A Power Supply Unit (PSU) is an internal hardware component. Despite the name, Power Supply Units (PSUs) do not supply systems with power – instead they convert it. Specifically, a power supply converts the alternating high voltage current (AC) into direct current (DC), and they also regulate the DC output voltage to the fine tolerances required for modern computing components. Most modern desktop personal computer power supplies conform to the ATX specification, which includes form factor and voltage tolerances. While an ATX power supply is connected to the mains supply, it always provides a 5-volt standby (5VSB) power so that the standby functions on the computer and certain peripherals are powered. ATX PSUs are turned on and off by a signal from the motherboard.